Such measurement apparatus are known with various sensor technologies and serve for detecting the contents of oil, hydrocarbons and oxidizable gases, for example in air or compressed air. Electrically heatable semiconductor oxide materials are frequently used, for instance, which, in the heated state, change their electrical resistance depending on the quantity of the hydrocarbons contained in the air.
Another method is the detection of hydrocarbons by means of elestors. To this end, the gas stream to be measured is conducted over a body of heated catalyst material in whose interior a heated coiled platinum filament is located. The hydrocarbon concentration can be detected through the change of electrical resistance of the heated and a second coiled platinum filament, which is due to the heat of combustion of the hydrocarbon content on the catalyst.
The use of flame ionization detectors is also known. In such devices, the hydrocarbons are burned in a gas stream and the change in the voltage between two electrodes in the flame is measured.
Another method is the detection of the hydrocarbon concentration by means of photoionization. In the process, the hydrocarbons are irradiated with ultraviolet light. The amount of energy of the light has to be so high in this case that electrons are forced out of the hydrocarbon. Their number can be measured by means of electrodes.
The above-mentioned methods are particularly suitable for the detection of higher concentrations in oxidizable gases; however, the detection of lower concentrations in the lower μg/Nm3 range or the ppb range in a reliable manner is not possible.
The measured values generated by means of photoionization detectors only allow for indirect conclusions to be drawn with regard to the measured amount of substance because the measured values are also dependent on the atomic structure of the compound and vary rather considerably even given the same empirical formulas. However, provided the compound to be measured is constant, known and, if possible, also uniform, the concentration of the hydrocarbon content can be measured relatively reliably. However, the measuring accuracy is reduced as the hydrocarbon concentration decreases. In particular, the influence of the humidity content of the air rises in this case. As the hydrocarbon content decreases, the influence of air humidity becomes increasingly larger; measurements of hydrocarbon contents in the lower mg/Nm3 range, and in particular in the μg/Nm3 range, cannot be carried out with sufficient accuracy.
Different threshold values of the oil content are required for different applications of compressed air. Oil contents consist of drop-like oil aerosols and of oil vapors. Oil aerosols and oil vapors can be partially or largely eliminated from the compressed air stream by means of various methods. However, a realtime measurement of oil in compressed air is a hitherto unsolved problem.